There are millions of HTML authors on the Web and they all know more or less about HTML. Some newbies write simple HTML pages and are able to do an okay job without knowing very much. Other authors are more adventurous and experiment with proprietary extensions to HTML, or with active content mechanisms, or with other kinds of code, some of which is actually supported by HTML. Needless to say, if you don't experiment with new things you will never learn new things, however if you don't know how to use the basic tools, experimenting with new stuff is more likely to produce extremely confusing results which will very likely vary widely depending on which browser you use to look at the code. Here are some of the more common errors I have seen in my role as a professional internet software tester.

This is an example of a document containing two links, the first one of which has a missing quotation mark. As you can see, the link appears to display correctly, but only one link displays and, when clicked, does not load the document correctly. Both Netscape and Internet Explorer will display the URL for the link in the status bar at the bottom of the window when your mouse-pointer is directly over the link. If you look in the status bar of your browser window while holding your cursor over the link in the first example it will give you a very clear idea of why the document doesn't load correctly. This is an example of the same document only the first link is missing the </A> (end anchor) tag to close the first link. In this example the link does not display correctly, however, when clicked, the document loads correctly. It is instructive to look at the source of these two documents to see where the errors occur in the actual HTML code.